The etiology of transsexualism, meaning the cause or causes of transsexualism has long been an area of interest for many transsexual people, physicians, psychologists, other mental health professionals, and family members and friends of transsexual people. Currently, there is no scientifically proven cause of transsexualism. For many years, many people assumed that transsexualism was a psychological/emotional disorder caused by psychological factors. More recently, research has suggested that the cause of transsexualism is rooted in biology, and a large segment of the medical profession has come to view transsexualism as a physiological condition rather than a psychological one [How to reference and link to summary or text]. However, as of 2006, physiological causes of transsexualism have not been proven.

Experience with individuals who were sexually reassigned at birth, in order to correct deformities such as those caused by accidental castration or intersex conditions, suggests strongly that one's mental gender identity is not determined by the environment they are raised in. Individuals born male but raised as female, or vice versa, often show the same signs of gender dysphoria as transsexual people; rejecting their assigned gender. One notable example was David Reimer. Reimer later suffered depression and heroin addiction in adulthood, which has been taken as proof of gender essentialist biological theories; however, critics of the case study note that his twin brother suffered from the same conditions, and that their depressive moods were likely genetic in origin.

Reparative therapy, which is usually aimed at gay and lesbian people, has also been applied to transsexual and transgender people. The Kinsey scale once expressed a view of transsexualism as an extreme form of homosexuality; the medical community now rejects this part of Kinsey's theory. Reparative therapy is generally ineffective for transsexual and transgender people as well as gay and lesbian people. [2] Even though many major medical and psychological associations have condemned reparative therapy as not only ineffective, but actually harmful,[3] it continues to be advocated as a treatment for both homosexual and transsexual people by various organizations in the Western World, often with ties to the conservative Christian movement or other conservativereligious movements.

However, for certain transsexual persons, therapies aimed at resolving gender conflicts, other than somatic treatments to reassign physical sex, may be effective and useful. Some people may have milder conflicts between their gender identity and physical sexual characteristics. These people may not wish to pursue sex reassignment therapy, but may seek care to help deal with the conflicts they face. If individuals express the desire for psychological care without plans for sex reassignment surgery, supportive and psychoeducational counseling may be helpful. Additionally, some transsexual people who may have a significant lifelong conflict between gender identity and their sexed body may present for care without requesting SRS. Their reasons for forgoing transition and/or SRS may include family and/or professional concerns, perceptions of the difficulty of transition, fear of loss of social standing or role, religious beliefs, political standpoints, real or perceived inability to finance transition, and advanced age or chronic medical problems, which may, in some cases, be considered medical contraindications to hormone therapy and/or sex reassignment surgery. These individuals often seek alternative methods with which they can improve their functional status, promote acceptance of their gender identity as valid, and ameliorate mood symptoms caused by gender conflict, through psychotherapy, and sometimes with medications. Additionally, these individuals sometimes benefit from partial transition options, such as low dose hormone therapy, validation of the patient's desire to dress and live partially in the gender role consistent with their gender identity, or even simply allowing the person a safe outlet to express themselves as members of their target gender. These options can provide a great deal of comfort to individuals who, for any reason, choose not to fully transition. [How to reference and link to summary or text] Such therapies may also benefit people who manifest transsexual-like symptoms but who are not diagnosable as "genuine" transsexual people.

Possible physical causes

A point of empirical evidence that a physical cause may be at hand in transsexualism, is that in the most common accounts of their childhood, transsexual (and also many other transgender) people claim to have had feelings of being either a girl or boy for as long as they are able to remember. As andectodal evidence, it is important to take it with a point of scepticism. However, the medical field for a long time had expected and required accounts such as this in order to receive access to medical treatment.

One study[4] gives strong evidence that transsexualism is based in structural and neurochemical similarities between the brains of transsexual people and brains typical of their gender identity; this study has been alleged to have numerous flaws [How to reference and link to summary or text]. A second study has since replicated the results of the first study and included controls to help eliminate many of the alleged flaws, particularly the study included two individuals who had cross-gender hormone secreting cancers which were significant enough to have caused significant cross-gender development in both individuals.[5] Although transsexualism manifests itself as an anomaly in brain structure in which transsexual people have a neuron density in the central subdivision of the Bed Nucleus of the Stria Terminalis region (BSTc), similar to members of their target gender, it is not known whether this is a cause, consequence, or simply a manifestation of transsexualism.

As of December 2006, a new study funded by Ferring Pharmaceuticals was presented at the International Paediatric Endocrinology Symposium in Paris. This study[6] finds that prior to hormone replacement therapy, transsexual people have brain structures matching their genetic sex. According to this study, a transsexual person's brain restructures itself to become typically structured for the individual's target sex with the administration of hormone therapy.

Several animal studies have demonstrated that exposure to cross-sex hormones during certain stages of fetal development can reliably produce cross-sex behaviors in animals. In addition, twin studies have demonstrated a strong heritability of transsexualism.[7] This research provides additional evidence that transsexualism may be caused by genetics and in utero hormonal environment. There are several known cases of identical twins who were both transsexual, both male-to-female and female-to-male. [How to reference and link to summary or text]

A recent study from Germany provides additional evidence of a physiological basis for transsexualism. The study found a correlation between digit ratio and male-to-female transsexualism. Male-to-female transsexual people were found to have a higher digit ratio than control males, but one that was comparable to control females. Because digit ratio is known to be directly related to prenatal hormone exposure, this tends to support theories linking such to male-to-female transsexualism.[8]

There is also evidence from transsexual people born between the 1930s and 1970s that exposure to a synthetic estrogen known as diethylstilbestrol(DES), routinely used at the time to prevent miscarriage and treat morning sickness, may have contributed to disrupting the hormonal balance within the womb. Evidence suggests that an unusually high percentage of physical males whose mothers were known to have taken this medication present as transgender or transsexual, either in childhood or in later life.[How to reference and link to summary or text]

Due to incidents of birth defects and other side effects, the use of DES and other synthetic estrogen compounds has been largely abandoned or replaced with natural estrogens.[9] Today, with widespread use of certain plastics and other substances, there are likely to be many environmental pollutants which closely mimic the chemical structures of the withdrawn drugs. This suggests that prenatal environmental factors could also influence the development of this condition.

A 2005 study found that prenatal exposure to phthalates reduced the anogenital distance in males.[10] Shorter anogenital distances were found to be associated with smaller penises, cryptorchidism, and lower levels of aggressiveness. Although no transsexual patients were included in this study, it suggests that environmental pollutants can affect sexual development in physical males.

Objection against research of transsexuals

Many scholars of gender theory, professionals who work with transsexual people, and transsexual and transgender people themselves, contest the very rationale of searching for a cause of transsexualism. An assumption behind this quest for a cause is that gender dimorphism (the idea that there are only two discrete, well-defined genders) is an established fact. Critics cite, among other things, historiographic and anthropological findings pointing to the fact that different cultures had diverse concepts of gender, some of them including three or more genders (see two-spirit, xanith, and hijra for examples.) Historically speaking though, the binary gender model has been the most prevalent, and the "third" gender has been, more or less, a "curiosity", or its members have formed an underclass.

One argument against the search for a cause of transsexualism is that it assumes a priori the legitimacy of a normative gender identity, i.e. gender identity congruent with the external genitalia. This, affirm the critics, is an unproven contention. Historical research shows that the relation between genitals and gender identity changes across cultures. Assuming a priori that variant gender identity is anomalous (and therefore that its causes should be investigated) distorts science's view of gender and contributes to the stigmatization of gender non-conformists.

Additionally, many people do not consider transsexualism to be a disease or disorder.[11] It should also be noted that the search for a physiological cause of transsexualism is in many ways similar to the search for a physiological cause of homosexuality. Many consider such research to be irrelevant, because they feel that, even if such a cause were established, it would not promote social acceptance of transsexual people, which is, for most transsexual people, the primary reason behind this quest for a physiological cause of their condition. Trasek, a Finnish organization for the transgender and intersex populations, suggested referring to transsexualism as a condition requiring medical intervention, similar to childbirth.